A reaction between a hydrocarbon such as methane or propane and steam or carbon dioxide is called a reforming reaction, which has been industrially carried out as means for obtaining a synthesis gas (hydrogen and carbon monoxide) or hydrogen used as fuel for fuel cells. A catalyst containing a noble metal such as nickel, ruthenium, rhodium, or platinum as an active component is generally used for the reforming reaction in order to accelerate the reaction. Since the reforming reaction is an endothermic reaction, a high temperature of 700° C. or more is generally required.
In recent years, a selectively permeable membrane reactor (membrane reactor) which allows reaction and separation to occur at the same time has been used to produce hydrogen or the like through the reforming reaction and to separate the reaction product. The selectively permeable membrane reactor includes a catalyst for accelerating a chemical reaction and a selectively permeable membrane exhibiting selective permeability, and allows a chemical reaction using the catalyst and separation of the reaction product using the selectively permeable membrane to occur at the same time. Therefore, the selectively permeable membrane reactor has a compact device configuration and requires only a small installation area. Moreover, since the reaction product passes through the selectively permeable membrane and is removed from the reaction system, the equilibrium of the chemical reaction shifts toward the production side, whereby the reaction can occur at a lower temperature. This reduces the consumption of energy during the reaction and prevents deterioration of the constituent material of the reactor.
As the reforming reaction using the selectively permeable membrane reactor, a steam reforming reaction of methane (see non-patent document 1, for example) and a carbon dioxide reforming reaction of methane (see non-patent document 2, for example) have been carried out and showed a conversion rate equal to or higher than the equilibrium.
Non-patent document 1: Eiichi Kikuchi, Catalysis Today, 25 (1995) 333-337
Non-patent document 2: Eiichi Kikuchi, Sekiyu Gakkaishi, 39, (5) (1996) 301-313
In a carbon dioxide reforming reaction of a hydrocarbon such as methane, the catalyst is significantly inactivated due to deposition of coke (coking) during the reaction. In particular, since coking occurs to a large extent when using the selectively permeable membrane reactor in comparison with the case of using a normal reactor, the conversion rate significantly decreases within a short time, thereby making it difficult to stably carry out the reforming reaction over a long time. In recent years, effective utilization of a biomass gas and a low-quality natural gas field has been demanded from the viewpoint of reducing carbon dioxide. Therefore, it is an important technical subject to efficiently carry out a reforming reaction using a selectively permeable membrane reactor utilizing a mixed gas containing a hydrocarbon obtained from the above source and carbon dioxide.